Historically, simple backlight devices included only three main components: light sources or lamps, a back reflector, and a front diffuser. Such systems are still in use for general purpose advertising signs and for indoor lighting applications.
Over recent years, refinements have been made to this basic backlight design by adding other components to increase brightness or reduce power consumption, increase uniformity, and/or reduce thickness. The refinements have been fueled by demands in the high-growth consumer electronics industry for products that incorporate liquid crystal displays (LCDs), such as computer monitors, television monitors, mobile phones, digital cameras, pocket-sized MP3 music players, personal digital assistants (PDAs), and other hand-held devices. Some of these refinements, such as the use of solid light guides to allow the design of very thin backlights, and the use of light management films such as linear prismatic films and reflective polarizing films to increase on-axis brightness, are mentioned below in connection with further background information on LCD devices.
Although some of the above-listed products can use ordinary ambient light to view the display, most include a backlight to make the display visible. In the case of LCD devices, this is because an LCD panel is not self-illuminating, and thus is usually viewed using an illumination assembly or backlight. The backlight is situated on the opposite side of the LCD panel from the viewer, such that light generated by the backlight passes through the LCD to reach the viewer. The backlight incorporates one or more light sources, such as cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs), and distributes light from the sources over an output area that matches the viewable area of the LCD panel. Light emitted by the backlight desirably has sufficient brightness and sufficient spatial uniformity over the output area of the backlight to provide the user with a satisfactory viewing experience of the image produced by the LCD panel.
LCD panels, because of their method of operation, utilize only one polarization state of light, and hence for LCD applications it is important to know the backlight's brightness and uniformity for light of the correct or useable polarization state, rather than simply the brightness and uniformity of light that may be unpolarized. In that regard, with all other factors being equal, a backlight that emits light predominantly or exclusively in the useable polarization state is more efficient in an LCD application than a backlight that emits unpolarized light. Nevertheless, backlights that emit light that is not exclusively in the useable polarization state, even to the extent of emitting randomly polarized light, are still fully useable in LCD applications, since the non-useable polarization state can be easily eliminated by an absorbing polarizer provided at the back of the LCD panel.
LCD devices can generally be considered to fall within one of three categories, and backlights are used in two of these categories. In a first category, known as “transmission-type”, the LCD panel can be viewed only with the aid of an illuminated backlight. That is, the LCD panel is configured to be viewed only “in transmission”, with light from the backlight being transmitted through the LCD on its way to the viewer. In a second category, known as “reflective-type”, the backlight is eliminated and replaced with a reflective material, and the LCD panel is configured to be viewed only with light sources situated on the viewer-side of the LCD. Light from an external source (e.g. ambient room light) passes from the front to the back of the LCD panel, reflects off of the reflective material, and passes again through the LCD on its way to the viewer. In a third category, known as “transflective-type”, both a backlight and a partially reflective material are placed behind the LCD panel, which is configured to be viewed either in transmission if the backlight is turned on, or in reflection if the backlight is turned off and sufficient ambient light is present.
Backlights described in the detailed description below can generally be used both in transmission-type LCD displays and in transflective-type LCD displays.
Besides the three categories of LCD displays discussed above, backlights can also be considered to fall into one of two categories depending on where the internal light sources are positioned relative to the output area of the backlight, where the backlight “output area” corresponds to the viewable area or region of the display device. The “output area” of a backlight is sometimes referred to herein as an “output region” or “output surface” to distinguish between the region or surface itself and the area (the numerical quantity having units of square meters, square millimeters, square inches, or the like) of that region or surface.
In “edge-lit” backlights, one or more light sources are disposed—from a plan-view perspective—along an outer border or periphery of the backlight construction, generally outside the area or zone corresponding to the output area. Often, the light source(s) are shielded from view by a frame or bezel that borders the output area of the backlight. The light source(s) typically emit light into a component referred to as a “light guide”, particularly in cases where a very thin profile backlight is desired, as in laptop computer displays. The light guide is a clear, solid, and relatively thin plate whose length and width dimensions are on the order of the backlight output area. The light guide uses total internal reflection (TIR) to transport or guide light from the edge-mounted lamps across the entire length or width of the light guide to the opposite edge of the backlight, and a non-uniform pattern of localized extraction structures is provided on a surface of the light guide to redirect some of this guided light out of the light guide toward the output area of the backlight. (Other methods of gradual extraction include using a tapered solid guide, wherein the sloping top surface causes a gradual extraction of light as the TIR angle is, on average, not reached by greater numbers of light rays as the light propagates away from the light source.) Such backlights typically also include light management films, such as a reflective material disposed behind or below the light guide, and a reflective polarizing film and prismatic BEF film(s) disposed in front of or above the light guide, to increase on-axis brightness.
In the view of Applicants, drawbacks or limitations of existing edge-lit backlights include: the relatively large mass or weight associated with the light guide, particularly for larger backlight sizes; the need to use components that are non-interchangeable from one backlight to another, since light guides must be injection molded or otherwise fabricated for a specific backlight size and for a specific source configuration; the need to use components that require substantial spatial non-uniformities from one position in the backlight to another, as with existing extraction structure patterns; and, as backlight sizes increase, increased difficulty in providing adequate illumination due to limited space or “real estate” along the edge of the display, since the ratio of the circumference to the area of a rectangle decreases linearly (1/L) with the characteristic in-plane dimension L (e.g., length, or width, or diagonal measure of the output region of the backlight, for a given aspect ratio rectangle).
In “direct-lit” backlights, one or more light sources are disposed—from a plan-view perspective—substantially within the area or zone corresponding to the output area, normally in a regular array or pattern within the zone. Alternatively, one can say that the light source(s) in a direct-lit backlight are disposed directly behind the output area of the backlight. Because the light sources are potentially directly viewable through the output area, a strongly diffusing plate is typically mounted above the light sources to spread light over the output area, to veil the light sources from direct view. Again, light management films, such as a reflective polarizer film, and prismatic BEF film(s), can also be placed atop the diffuser plate for improved on-axis brightness and efficiency. Large area LCD applications tend to use direct-lit backlights because they are not constrained by the 1/L limitation of edge-lit backlights, and because of the weight associated with solid light guides.
In the view of Applicants, drawbacks or limitations of existing direct-lit backlights include: inefficiencies associated with the strongly diffusing plate; in the case of LED sources, the need for large numbers of such sources for adequate uniformity and brightness, with associated high component cost and heat generation; and limitations on achievable thinness of the backlight beyond which light sources produce non-uniform and undesirable “punchthrough”, wherein a bright spot appears in the output area above each source.
In some cases, a direct-lit backlight may also include one or some light sources at the periphery of the backlight, or an edge-lit backlight may include one or some light sources directly behind the output area. In such cases, the backlight is considered “direct-lit” if most of the light originates from directly behind the output area of the backlight, and “edge-lit” if most of the light originates from the periphery of the output area of the backlight.